APU FUEL SYSTEM -FUEL CLUSTER -FUNCTIONAL DESCRIPTION
The mode select valve also acts as a flow-sensitive device to control the pressure to the pressurizing shutoff valve. During engine start, the gear pump output pressure goes directly to the pressurizing shutoff valve. When the APU speed is 59 percent RPM, the gear pump output pressure is high enough to move the mode select valve. The mode select valve then sends boost pump pressure to the pressurizing shutoff valve.
Pressure Regulator
The pressure regulator keeps the fuel pressure at 325 psi more than the boost pump pressure for the IGV and SCV actuators and the fuel metering valve.
Bypass Valve
The bypass valve sends all fuel not needed by the fuel metering valve back to the return line. This keeps the pressure differential across the fuel metering valve.
Fuel Metering Valve
The fuel metering valve is an electro-hydraulic servo-valve. It controls the amount of fuel supplied to the combustion chamber.
An LVDT measures the position of the metering valve and sends a feedback signal to the APUC.
Pressurizing Shutoff Valve
The pressure-operated pressurizing shutoff valve permits flow from the fuel metering valve to the fuel divider when the valve is open. When the valve is closed, flow from the fuel metering valve goes to the return line through the shutoff relief valve.
During APU start, the valve opens when the gear pump output pressure is 129 psid or more. During APU shutdown, high pressure from the fuel shutoff solenoid valve closes the valve. When there is no pressure, a spring keeps the valve closed.
Fuel Shutoff Solenoid Valve
The fuel shutoff solenoid valve controls the pressurizing shutoff valve during APU start and shutdown.
During APU start, the APUC energizes the solenoid of the fuel shutoff solenoid valve at 7 percent RPM. The valve then closes and removes pressure to the pressurizing shutoff valve.
During APU shutdown, the APUC removes the power from the solenoid of the fuel shutoff solenoid valve. The valve then opens to permit high pressure fuel to close the pressurizing shutoff valve. This stops the flow of fuel to the combustion chamber.
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APU FUEL SYSTEM -FUEL CLUSTER -FUNCTIONAL DESCRIPTION
Shutoff Relief Valve
The shutoff relief valve permits the metered fuel to flow to the return line when the pressurizing shutoff valve is closed. The shutoff relief valve opens at 300 psid to keep pressure on the fuel metering valve.
Shutoff Restrictor
The shutoff restrictor permits the pressure on the pressurizing shutoff valve to bleed to return when the fuel shutoff solenoid valve is closed.
Flow Divider
The flow divider is a pressure-operated valve that controls fuel flow to the secondary manifold. During APU start, the flow divider opens at approximately 50 percent RPM. This permits fuel flow to the secondary manifold and greater flow to the combustion chamber when it is necessary. The flow divider stays open during APU operation.
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APU FUEL SYSTEM -FUEL CLUSTER -FUNCTIONAL DESCRIPTION
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APU FUEL SYSTEM -MANIFOLDS AND NOZZLES
Fuel Manifolds
A primary fuel manifold and a secondary fuel manifold supply fuel from the fuel cluster to the fuel nozzles.
The manifolds are insulated flexible lines and are on the combustion section.
Both manifolds supply fuel when the APU is running. Only the primary manifold supplies fuel during the first part of APU start at low RPM.
Fuel Nozzles
Fourteen fuel nozzles atomize and inject fuel into the engine combustion section. The fuel nozzles are equally spaced around the combustion section.
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777 AIRCRAFT MAINTENANCE MANUAL
APU FUEL SYSTEM -MANIFOLDS AND NOZZLES
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APU FUEL SYSTEM -FUNCTIONAL DESCRIPTION
General
The APU controller controls the APU fuel system. The APU fuel system supplies fuel for combustion and is a pressure source for the IGV and SCV actuators.
Fuel Supply
Fuel system ac boost pumps or the APU dc fuel pump supplies fuel through the APU fuel shutoff valve. The shutoff valve opens when the APU selector is ON. The dc pump operates when ac power is not available. When ac power becomes available, the left forward boost pump turns on and the dc pump turns off.
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